Integrated Financial Management Information Systems (IFMIS) are meant to improve fiscal control, transparency, and the quality of public sector decision-making. In practice, however, a recurring tension remains: the system may be fully operational for routine transactions, yet its outputs are often not the ones trusted when major budgeting decisions are made. This paper examines that gap through the idea of strategic underuse, where a system is formally in place and actively used, but is quietly sidelined in high-level decision forums. Drawing on an interpretive qualitative case study of Kenya’s public sector budgeting environment, the study analyzes documentary evidence from 2016 to 2025, including audit reports, parliamentary proceedings, Treasury circulars, and budget publications. Using process tracing and the Gioia method, the paper develops a process model showing how shadow systems such as spreadsheets, reconciled summaries, and manually prepared briefing tools emerge, stabilize, and eventually become taken for granted. The findings show that these shadow systems do not arise simply because of technical weakness. Rather, they grow out of a persistent mismatch between what IFMIS is designed to do and what strategic budgeting work actually demands under conditions of time pressure, accountability, and institutional scrutiny. Over time, repeated reliance on these alternative tools gives them credibility and turns them into the practical “system of truth,” even while IFMIS remains the formal system of record. The paper contributes to information systems research by showing that underuse is not just a failure of adoption, but a gradual institutional process through which alternative infrastructures gain authority in practice.